Experimental and theoretical investigation of the oxidative carbonylation of toluene to toluic acid catalyzed by palladium(II) in the presence of vanadium and molecular oxygen

The mechanism and kinetics of the liquid-phase, oxidative carbonylation of toluene to toluic acid over Pd(II) in the presence of trifluoroacetic acid (TFAH), trifluoroacetic anhydride (TFAA), and molecular oxygen were investigated through a combination of experimental and theoretical approaches. The experimental results are consistent with the previously proposed mechanism for the oxidative carbonylation of arenes. The reaction is initiated by coordination of toluene to the Pd(II) complex and activation of a CH bond in the benzene ring. This initial step becomes rate limiting when a sufficiently high (NH4VO3)/Pd ratio is used for the reoxidation of Pd(0) to Pd(II). Both processes are found to be dependent on the electron withdrawing and donating capability of the anionic ligands. Overall catalyst activity peaks for ligands of intermediate basicity, and diminishes for both more and less basic ligands. Theoretical analysis of the coordination of toluene and activation of the CH bond on the benzene ring reveals that the basicity of the ligands affects the two processes in opposite ways. Weakly basic ligands promote the coordination of toluene but have the opposite effect on the activation of the CH bond. The tradeoff in these two effects leads to a maximum in the apparent rate coefficient with pKa of the conjugate acid of the anionic ligands. The absence of significant product stereoselectivity is due to a lack of steric hindrance in the binding of toluene to the Pd(II) complex.

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► A combined experimental and theoretical investigation of the oxidative carbonylation of toluene over Pd(II) is performed.
► The mechanism is found to proceed via electrophilic binding toluene and subsequent CH bond activation.
► The role of Pd's ligand structure is investigated, with intermediate acidity ligands producing the most active catalysts.
► The Pd system is compared against previous work with an analogous Rh system.